The CACHE INDEX statement assigns
table indexes to a specific key cache. It is used only for
MyISAM tables. After the indexes have been
assigned, they can be preloaded into the cache if desired with
LOAD INDEX INTO
CACHE.

The following statement assigns indexes from the tables
t1, t2, and
t3 to the key cache named
hot_cache:

The syntax of CACHE INDEX enables
you to specify that only particular indexes from a table should
be assigned to the cache. The current implementation assigns all
the table's indexes to the cache, so there is no reason to
specify anything other than the table name.

The key cache referred to in a CACHE
INDEX statement can be created by setting its size
with a parameter setting statement or in the server parameter
settings. For example:

By default, table indexes are assigned to the main (default) key
cache created at the server startup. When a key cache is
destroyed, all indexes assigned to it become assigned to the
default key cache again.

Index assignment affects the server globally: If one client
assigns an index to a given cache, this cache is used for all
queries involving the index, no matter which client issues the
queries.

In MySQL 5.5, this statement is also supported for
partitioned MyISAM tables. You can assign one
or more indexes for one, several, or all partitions to a given
key cache. For example, you can do the following:

Creates a partitioned table with 4 partitions; these
partitions are automatically named p0,
..., p3; this table has an index named
i on column c1.

Creates 2 key caches named kc_fast and
kc_slow

Assigns the index for partition p0 to the
kc_fast key cache and the index for
partitions p1 and p3
to the kc_slow key cache; the index for
the remaining partition (p2) uses the
server's default key cache.

If you wish instead to assign the indexes for all partitions in
table pt to a single key cache named
kc_all, you can use either one of the
following 2 statements:

CACHE INDEX pt PARTITION (ALL) IN kc_all;
CACHE INDEX pt IN kc_all;

The two statements just shown are equivalent, and issuing either
one of them has exactly the same effect. In other words, if you
wish to assign indexes for all partitions of a partitioned table
to the same key cache, then the PARTITION
(ALL) clause is optional.

When assigning indexes for multiple partitions to a key cache,
the partitions do not have to be contiguous, and you are not
required to list their names in any particular order. Indexes
for any partitions that are not explicitly assigned to a key
cache automatically use the server's default key cache.